System and method for dendritic web solar cell shingling

ABSTRACT

A dendritic web solar cell shingled array comprises at least two dendritic web solar cells. A first cell overlaps a portion of a second cell such that a back contact of the first cells interconnects with a top contact of the second cell. The cells are less than 150 microns thick, allowing a direct connection between the back contact and top contact of the two cells without the use of a visible busbar. The cells may be shingled together using soldering material and/or electrically conductive adhesives.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates generally to solar cells, and moreparticularly provides a system and method for dendritic web solar cellshingling.

[0003] 2. Description of the Background Art

[0004] Solar cells convert sunlight into electricity through aphotovoltaic process and can be used in small installations, such as inwatches or calculators, to provide small amounts of electrical power. Inanother embodiment, solar cells can be grouped into large arrays ormodules, which may contain thousands of individual cells, to convertsunlight into large amounts of electrical power to provide power tohomes or industry.

[0005] A conventional solar cell comprises several layers, including ann-type silicon layer generally doped with phosphorus, which faces thesunlight; a p-type silicon layer located beneath the n-type siliconlayer and generally doped with boron; an antireflective coating on topof the n-type silicon to reduce reflection of sunlight; and twoelectrical contact layers made of conducting material. The solar cellmay also be laminated with ethylene vinyl acetate and have a glass layerto protect the cell from the environment, i.e., from airborne particles,snow, rain, etc.

[0006] The first electrical contact layer includes a back contact layerthat is generally made of a conducting material and covers the entireback surface area of the cell. The second contact layer is located onthe face of the cell on top of n-type silicon layer facing the sun andis also made of a conducting material. The second electrical contactlayer is conventionally arranged in a grid-type pattern such that thesecond electrical contact layer does not cover the entire face of thecell since sunlight cannot typically pass through the second electricalcontact layer.

[0007] In order to increase electrical power output, a plurality ofsolar cells may be grouped together into an array. Increasing thesurface area available by increasing the number of solar cells increasesthe amount of electrical power that can be produced. In a conventionalarray, there is usually a gap between solar cells to allow for circuitryfor coupling the cells together. This gap reduces the proportion oftotal solar cell surface area in a solar cell array, thereby reducingthe proportion of solar cell surface area exposed to sunlight thatproduces electrical power. In addition, an interconnect tab that goesfrom a back of one cell to a top of another cell may be used to couplecells together. This interconnect tab technique for coupling cells leadsto undesirable stress on the cells.

[0008] A conventional solution for increasing the proportion of solarcell surface area in a solar array is to remove the gap by shinglingsolar cells. The shingling architecture is implemented by applyingsoldering material to the bottom of the back contact layer on a firstsolar cell and/or the top of a busbar of the second contact layer on theface of a second solar cell. The contact layers of the two cells arethen soldered together, forming a continuous conducting medium betweenthe contact layers of the two solar cells, thereby allowing electricalcurrent to flow from the first solar cell to the second solar cell.

[0009] However, due to the thickness of the solar cells (usually 300-600microns), a large height differential can develop between the first andlast solar cells in a long series of shingled cells in a solar array,thereby leading to difficulty in handling and installing the solararray. Further, the busbars of a face contact layer on the solar cellblock sunlight, thereby reducing solar cell area available forelectrical power production. In addition, the busbars may expand at adifferent rate than the rest of the solar cell due to thermal heating,thereby possibly disconnecting one solar cell from another.

[0010] Accordingly, a new solar cell array is highly desirable that mayallow for increased solar cell area without creating a heightdifferential between a first and last solar cell in a solar cell array.

SUMMARY

[0011] The present invention provides a shingled solar cell arraysystem. The system comprises solar cells made of dendritic web siliconsubstrates arranged in series in a solar cell shingling array. The solarcells have a thickness of about 80-150 microns. The solar cells arecoupled together by soldering a back contact layer of one cell to a face(top) contact layer (or grid) of a second solar cell. In anotherembodiment of the invention, the cells are coupled together usingelectrically conductive adhesives. Further, unlike conventional solarcells, the solar cells of an embodiment of the present invention aremuch thinner, allowing for easier handling of the array due to a lowheight differential between the first and last cells, and lack exposedbusbars, thereby preventing possible problems relating to uneven thermalexpansion of the different materials of the solar cell.

[0012] The present invention further provides a method for solar cellshingling. The method comprises generating a dendritic web solar cellhaving a thickness of under 150 microns; placing solder material on aface/top contact of a first cell and/or on a back contact layer of asecond cell; aligning the cells in series such that the face contactlayer of the first cell is in place with the back contact layer of thesecond cell; and applying heat to the cells such that the soldermaterial bonds the two cells together.

[0013] The system and method may advantageously increase solar cellarray efficiency by shingling dendritic web solar cells.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a diagram illustrating a solar cell array in accordancewith an embodiment of the present invention;

[0015]FIG. 2 is a cross section of the solar cell array of FIG. 1;

[0016]FIG. 3 is a perspective view of a solar cell from the array ofFIG. 1;

[0017]FIG. 4 is a perspective view of two interconnected solar cellsfrom the array of FIG. 1;

[0018]FIG. 5 is a flowchart of steps for solar cell shingling to formthe array of FIG. 1; and

[0019]FIG. 6 depicts an I-V curve, which shows a light energy toelectrical energy conversion efficiency of 12.2% for a shingled solarcell array having twelve dendritic web silicon substrate solar cellsarranged in series.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0020] The following description is provided to enable any personskilled in the art to make and use the invention, and is provided in thecontext of a particular application and its requirements. Variousmodifications to the embodiments will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other embodiments and applications without departing from thespirit and scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown, but is to be accordedthe widest scope consistent with the principles, features and teachingsdisclosed herein.

[0021]FIG. 1 is a diagram illustrating a solar cell array 100 inaccordance with an embodiment of the present invention. The solar cellarray 100 comprises four individual solar cells 120, 130, 140, and 150,which are made of dendritic web silicon and may be identical to eachother. In alternative embodiments, the solar cell array 100 may compriseany number of solar cells.

[0022] The solar cells 120, 130, 140, and 150 of solar cell array 100are arranged in a tile or shingle format such that the bottom of onecell overlaps the top of another cell. The overlapping area is generallyless than 10% of the total area of a solar cell. Due to the thin natureof the dendritic web silicon solar cells, the overlapping of the cellscauses a back contact layer of one cell to come into direct contact withthe top contact layer of the next cell, thereby allowing electricalcurrent to flow from the first cell to the next cell. Accordingly,exposed busbars, as used in conventional solar cells, are not required,thereby increasing available effective surface area as compared toconventional cells. However, a last cell in a series, such as cell 150,may have an exposed busbar 160. Further, an additional advantage of thesolar cell array 100 as compared to conventional arrays is that the thinnature of the solar cells 120, 130, 140 and 150 lead to less of heightdifferential between the first and last cells in the series array,leading to easier handling of the array 100.

[0023] In addition, as there is no need for a separate interconnectbetween cells, undesirable stress in thin cells is avoided as a tabconnecting the back of one cell to a front of a next cell is absent.Further, due to the lack of a tab between cells, resistance is loweredover a series of cells, thereby increasing power delivery. The layout ofthe solar cells will be discussed further in conjunction with FIG. 3 andFIG. 4 below.

[0024]FIG. 2 is a cross section of the solar cell array 100 of FIG. 1.As mentioned in conjunction with FIG. 1, the bottom of one solar celloverlaps the top of the next solar cell. For example, at junction 200the bottom of solar 140 overlaps the top of solar cell 130, therebyforming a connection between the top contact layer of cell 130 and thebottom contact layer of cell 140, as will be discussed further inconjunction with FIG. 4 below.

[0025]FIG. 3 is a perspective view of solar cell 130 from the solar cellarray 100 of FIG. 1. Solar cell 130 has a thickness of about 80 to 150microns as compared to 300 to 600 microns for conventional solar cells.Solar cell 130 comprises a p-type silicon layer 310 made of dendriticweb silicon crystal doped with boron or other suitable material; ann-type silicon layer 320 also made of dendritic web silicon crystaldoped with phosphorus or other suitable material disposed on top of thep-type silicon layer 310; a back or bottom contact layer 300 locatedbeneath the p-type silicon layer 310; and a top (or face) contact layer330 disposed on the n-type silicon layer 320. Solar cell 130 may alsocomprise an antireflective coating 340 disposed on top of the n-typesilicon layer 320, in which case, the top contacts 330 fire through theanti-reflective coating 340. The anti-reflective coating can be made ofsilicon nitride, titanium dioxide or other suitable materials andreduces reflection of photons from n-type silicon layer 320, therebyincreasing solar cell efficiency. The top contact layer 330 can be madeof silver ohmic contacts or other conducting material and the bottomcontact layer 300 may be of a silicon aluminum eutectic metal layer orother conducting material.

[0026] A further example of dendritic web silicon solar cells that canbe used in the present invention can be found in PCT Application No.PCT/US00/02609 entitled “An Aluminum Alloy Back Junction Solar Cell anda Process for Fabrication Thereof” published on Sep. 21, 2000 as WO00/55923, which is incorporated by reference.

[0027]FIG. 4 is a perspective view of two interconnected solar cells 130and 140 from the array 100 (FIG. 1). Solar cell 130 is interconnected tosolar cell 140 at junction 200. Due to the thin nature of dendritic websilicon solar cells, back contact 400 of solar cell 140 is in contactwith the top contacts 330 of solar cell 130, thereby allowing electricalcurrent to flow between cells without the need for thick screen printedbusbars or interconnection materials (such as tinned copper strips),which normally limit the solar cell surface area available for producingelectrical current. Further, the lack of interconnection materials mayremove some problems associated with uneven thermal expansion ofconventional solar cells.

[0028]FIG. 5 is a flowchart of steps for solar cell shingling to formthe solar cell array 100 of FIG. 1. At step 500, dendritic web solarcells are generated using techniques known in the art. For example, amethod of generating dendritic web solar cells is disclosed in WO00/55923. At step 510, soldering material is placed on the back contactlayer of one cell and/or on the face (top) contact layer of a secondcell along an edge of the cells. The soldering material may be placedalong the entire lengths of the solar cells. In an alternativeembodiment, electrically conductive adhesives may be used in place ofsoldering material.

[0029] At step 520, the solar cells are arranged in series in a tileformat so that the bottom of one solar cell overlaps the top of anothersolar cell. It is preferred to have not more than 10% of the surfacearea of any solar cell covered by another solar cell so as not to limitor waste the surface area available for electrical power production. Atstep 530, heat is applied to the soldering material or to the solarcells so that the soldering material bonds the solar cells together intoa string. It will be appreciated that the melting point of the solderingmaterial may be designed to be lower than the melting point of the cellsso that cells are not damaged when the cell/solder combination is heatedas a whole to melt the solder. It will be further appreciated that onlya portion of the solder need melt to fuse the cells together. Ifelectrically conductive adhesives are used in place of solderingmaterial, heat need not be applied and step 530 may be skipped.

[0030] At step 540, two or more solar cell strings may be connectedtogether, typically in parallel. At step 550, the interconnectedshingled solar cell strings are encapsulated in a lamination process,which is well known in the art. Typical layers in the laminate include atransparent protective cover, such as glass, a potting material, such asethylene vinyl acetate, the interconnected cell strings, and aprotective back layer, such as tedlar. In this way, the solar cellstrings 100 are protected from breakage and contaminants such asairborne particles, snow, rainwater, etc.

[0031]FIG. 6 depicts an I-V curve, which shows a light energy toelectrical energy conversion efficiency of 12.2% for a solar cell arrayhaving twelve laminated shingled dendritic web silicon crystal solarcells, each with 2.72 cm² exposed area, arranged in series. The arrayhas a fill factor of 0.782. The knee of the curve shows the maximumpower point P_(max) wherein V_(mp) is approximately 5.78 volts and Impis approximately 0.069 amps yielding a P_(max) of 0.0399 watts. Incomparison, a solar cell array in a non-shingled format using a set ofsimilar twelve solar cells has a conversion efficiency of only 10.9%because of a fill factor of 0.700.

[0032] The foregoing description of the preferred embodiments of thepresent invention is by way of example only, and other variations andmodifications of the above-described embodiments and methods arepossible in light of the foregoing teaching. For example, the p-typesilicon layer 310 may be doped with gallium or aluminum instead ofboron. The shingling method described can be used wherever positive andnegative contacts are on opposite sides of the solar cell. Theembodiments described herein are not intended to be exhaustive orlimiting. The present invention is limited only by the following claims.

What is claimed is:
 1. A shingled solar cell array, comprising: at leasttwo dendritic web silicon solar cells shingled together so that a bottomof one cell overlaps a top of a next cell, the cells each having athickness of less than about 150 microns.
 2. The shingled solar cellarray of claim 1, wherein less than 10% of the surface area of one cellis overlapped by the next cell.
 3. The shingled solar cell array ofclaim 1, wherein the at least two cells are interconnected via a backcontact layer of one cell to a top contact layer of the next cell. 4.The shingled solar cell array of claim 1, wherein the array is has ananti-reflective coating.
 5. The shingled solar cell array of claim 1,wherein the array is encapsulated with an encapsulation material.
 6. Theshingled solar cell array of claim 5, wherein the encapsulation materialincludes ethylene vinyl acetate.
 7. The shingled solar cell array ofclaim 1, wherein the shingled solar cell array has an interconnectbetween cells having a low resistance.
 8. The shingled solar cell arrayof claim 1, wherein the shingled solar cell array has an interconnectbetween cells lacking undesirable stress.
 9. A solar cell shinglingmethod, comprising: placing soldering material on a top contact layer ofa first dendritic web silicon solar cell, the first cell having athickness of less than about 150 microns; overlapping a second dendriticweb silicon solar cell over the soldering material, the second cellhaving a thickness of less than about 150 microns; applying heat to thefirst and second solar cells so that the cells bond.
 10. The method ofclaim 9, further comprising placing soldering material on the backcontact layer of the second cell.
 11. The method of claim 9, furthercomprising encapsulating the solar cells with an encapsulation material.12. The method of claim 11 wherein the encapsulation material includesethylene vinyl acetate.
 13. The method of claim 9, wherein the first andsecond cells have an anti-reflective coating.
 14. The method of claim 9,wherein the second cell does not overlap more than 10% of the surfacearea of the first cell.
 15. A system for solar cell shingling method,comprising: means for placing soldering material on a top contact layerof a first dendritic web silicon solar cell, the first cell having athickness of less than about 150 microns; means for overlapping a seconddendritic web silicon solar cell over the soldering material, the secondcell having a thickness of less than about 150 microns; means forapplying heat to the first and second solar cells so that the cellsbond.
 16. A solar cell shingling method, comprising: placing solderingmaterial on a bottom contact layer of a first dendritic web siliconsolar cell, the first cell having a thickness of less than about 150microns; coupling the first cell with a second dendritic web siliconsolar cell such that a top contact layer of the second cell is incommunication with the soldering material, the second cell having athickness of less than about 150 microns; applying heat to the first andsecond solar cells so that the cells bond.
 17. A shingled solar cellarray, comprising: at least two dendritic web silicon solar cellsshingled together with electrically conductive adhesive so that a bottomof one cell overlaps a top of a next cell, the cells each having athickness of less than about 150 microns.
 18. A solar cell shinglingmethod, comprising: placing electrically conductive adhesive on a topcontact layer of a first dendritic web silicon solar cell, the firstcell having a thickness of less than about 150 microns; and overlappinga second dendritic web silicon solar cell over the adhesive so that thecells bond, the second cell having a thickness of less than about 150microns.